According to Federal Communications Commission (FCC), wireless communication performed using a frequency band for televisions is considered to be realized by a cognitive wireless communication terminal as a secondary user. Specifically, the cognitive wireless communication terminal is considered to be realized after the cognitive wireless communication terminal confirms that a white space is present (that is, no signal is present in operation). As a signal in operation, a primary signal such as a digital terrestrial television signal (DTT signal) can be exemplified.
Accordingly, when the cognitive wireless communication is performed, it is at least necessary to confirm that no primary signal is present in a predetermined space, time, or frequency band.
As technologies for confirming whether a primary signal is present, three technologies, that is, a technology relevant to a database (for example, a database for a television white space), a technology relevant to sensing, and a technology relevant to a cognitive pilot channel (CPC) can be mainly exemplified (for example, see Non-Patent Literature 1). Moreover, a cognitive wireless communication terminal as a secondary user can perform communication using the spectrum of an available frequency by confirming that no primary signal is present.
An object of the above-described technology is to prevent an unlicensed cognitive wireless communication terminal as a secondary user from generating interference, which may have an adverse influence, with respect to a licensed primary user. Avoidance of the interference is required not only for the communication performed using a television white space but also for communication performed using a white space.
Further, there is the following problem as the more recent problem with the television white space. When various cognitive wireless communication terminals (secondary users) are present within a certain area, a plurality of wireless networks are formed within the area in some cases. That is, the plurality of wireless networks coexist within the predetermined area. In this case, it is necessary to avoid the interference between the cognitive wireless communication terminals as well.
Therefore, a coexistent database (sensing database) is considered to be provided to avoid the interference when the plurality of wireless networks coexist. The coexistent database stores information regarding the secondary users. In this case, each user can understand the states of the networks by accessing the coexistent database. Moreover, each user can determine an operation to avoid the interference in accordance with the state of the network which the user understands.
However, when each user accesses the coexistent database, it takes that time (that is, delay occurs). Accordingly, a method different from the method of providing the coexistent database needs to be provided to avoid the interference between the networks. Moreover, when this method can be realized, the plurality of wireless networks can coexist without delay.